1. Field of the Invention
The subject invention relates to a process for manufacturing colorants. More particularly, it relates to a process for manufacturing non-volatile colorants and the resulting products which are suitable for use in ribbon, ball-point pen, and other inks. These colorants are manufactured from a by-product stream of alkali blue preparation.
2. Description of the Related Art
Although many of the triarylmethane dyestuffs have declined in importance in recent years, pararosaniline and its analogues have remained significant products, especially as intermediates in preparing colorants such as spirit blue and sulfonated derivatives, for example, soluble sky blue, and particularly alkali blue.
The precursors to these sulfonated products have been produced by a variety of methods, for example by the condensation of ring substituted or unsubstituted methylenedianilines (MDA) with aniline or substituted anilines such as the toluidines. These precursors thus have the general formula: ##STR1## wherein R.sup.1 may be, by way of example, hydrogen, C.sub.1 -C.sub.8 alkyl or alkenyl, halo, cyano, or alkoxy; X is an anion of valence n, for example an ion such as chloride, sulfate, or nitrate; n is a whole number from 1 to about 3; and R is hydrogen, lower alkyl, lower alkenyl, phenyl, substituted phenyl, naphthyl, or substituted naphthyl.
The simplest member of this series is pararosaniline (paramagenta, parafuchsine, CI 42500) where R and R.sup.1 are both hydrogen. Thus the structural formula is: ##STR2## Of course, as is well known, the structural form given above represents only one cannonical version of the actual structure which is a resonance hybrid.
Pararosaniline has been prepared by heating 4,4'-MDA with aniline, aniline hydrochloride, nitrobenzene, and ferric chloride; by heating aniline and p-toluidine and their hydrochlorides with iron or ferric chloride and nitrobenzene; and by oxidizing a mixture of aniline with AlCl.sub.3 and carbon tetrachloride. Modernly, however, the preferred method of manufacture is the air oxidation of 4,4'-MDA and aniline in excess aniline, using vanadium oxide or other suitable catalyst. Replacement of 4,4'-MDA in whole, or in part, with ring substituted MDA's such as 3,3'-dimethyl-4,4'-MDA or 2,2'-dimethyl-4,4'-MDA, or the replacement of aniline by substituted anilines such as the toluidines results in similar products having somewhat different color, shade, and tinctorial strength. Of course, N-methylated MDA's and aniline may also be utilized. The presence of the N-alkyl groups causes a greater bathochromic shift in the absorption spectrum due to the increased basicity of the dye base.
Pararosaniline and its analogues mentioned above are excellent candidates for ink colorants, particularly for ribbon inks. In these applications, the dyes are generally used in the form of salts, most often as salts of resinic acids such as oleic acid. Due to the methods of synthesis generally used, however, it has proven difficult to prepare such salts in relatively pure form.
Older syntheses of pararosanilines and similar compounds generally resulted in the formation of the practically colorless leuco base, which could easily be isolated as a filter cake and washed free of contaminants and unreacted anilines and toluidines. The leuco form could be converted to the carbinol form by oxidation with lead dioxide. Subsequent reaction in situ (the carbinol form is not reported to have been isolated) with acid yielded the dyestuff. The modern process, however, results in formation of the hydrochloride salt directly.
By conducting the reaction in the presence of excess aniline and eliminating the addition of acid during oxidation, the relatively unstable color base may be formed. Unfortunately, the color base cannot itself be reacted with resinic acids to form a commercial product because the resulting resinate contains large quantities of impurities, particularly aniline, which makes the product unsuitable for its use in ribbon inks and the like because their arylamine content would require the presence of poison labels. Such labels would make the product commercially unacceptable. The hydrochloride cannot be used to form the resinate, due to the fact that resinic acids, such as oleic acid, fail to react because the acid-base equilibrium lies far to the side of the original hydrochloride salt. Thus a method of preparation of pararosaniline family colorant resinates is desirable which would enable the economic removal of reaction by-products while allowing for a aniline-free product. Heretofore, such a process has not been available to the dye industry.